1. Field of the Invention
The invention concerns a method for controlling a vehicle power train containing a combustion engine and a continuously variable transmission, in particular under constant or decreasing vehicle velocity.
2. Description of the Related Art
Automatic transmissions, in particular transmissions a continuously variable transmission ratio, belt-driven conical-pulley transmissions, for example, are enjoying increasing popularity in motor vehicles. They not only allow improved driving comfort, but they also make reductions of fuel consumption possible, since the combustion engine is operated at the slowest possible speed, dependent on the power demand.
The object of the invention is to further reduce fuel consumption of such power trains as well as to reduce to a minimum the need for operating the brake pedal.
A first solution of that object is achieved through a method for controlling a vehicle power train containing a combustion engine and a continuously variable transmission, wherein the transmission ratio of the transmission and the power delivered by the combustion engine are controlled depending on the momentary vehicle velocity and a requested propulsive power. As the actual velocity of the vehicle increases over a desired velocity, the transmission ratio of the transmission is changed in the direction of maximum transmission ratio, and as the power decreases further the quantity of fuel fed to the engine is decreased.
Advantageously, during deceleration, the quantity of fuel fed to the engine is reduced to zero.
Also preferred is that when velocity increases and greater propulsive power is not requested, kinetic energy of the vehicle is loaded into an energy storage device by means of a recouperation unit positioned in the power train.
In a further embodiment of the method according to the invention, the transmission ratio of the transmission is changed in the direction of a lower transmission ratio when the vehicle velocity increases during an overrun condition while the fuel supply is shut off.
Also preferred is to carry out the method in such a way that a vehicle brake is activated when the actual velocity increases while the transmission is shifted in the direction of a lower transmission ratio.
A further solution of the object on which the invention is based is achieved with a method for controlling a vehicle drive train containing a combustion engine and a continuously variable transmission, wherein the transmission ratio of the transmission is controlled in such a way that the combustion engine runs at a desired speed that depends on the momentary vehicle velocity and the requested propulsive power. The fuel supply to the combustion engine is interrupted if the combustion engine is running at a speed that is higher than a target speed. According to the present invention, the desired target speed is increased depending on predetermined operating conditions. As a result, the fuel supply can remain shut off over a greater operating range.
For example, the desired engine speed can be increased when deceleration fuel cutoff is activated, if the transmission is not shifted to its lowest transmission ratio.
Alternatively, or in addition, the desired engine speed can be increased if a vehicle brake is activated.
Also preferred is for the increase of the desired engine speed to be canceled more quickly when the requested propulsive power is increased than when the activation of the brake is ended.
The amount by which the desired engine speed is increased depends advantageously on operating parameters of the power train.
It is also advantageous to limit the gradients of the change to the desired engine speed.
A further solution of the object of the invention is achieved with an apparatus for controlling a vehicle drive train containing a combustion engine and a continuously variable transmission, which includes sensors for detecting at least the position of an accelerator pedal, the vehicle velocity, and the transmission ratio of the transmission, actuators for adjusting a power setting element of the combustion engine, the transmission ratio of the transmission and the supply of fuel to the combustion engine, and a control unit which controls the actuators depending on output signals from the sensors to carry out at least one of the above-mentioned procedures.